Identification of All Probable Causes and Development of Corrective Actions
Basic Principle When Implementing Measures
The project team conducted detailed analyses of every one of the 100 or so probable causes which were identified in the investigations, and confirmed that about 20 of them were "theoretically possible, but realistically impossible" or that "they had already been corrected by relevant measures." The project team then implemented measures for the remaining 80 or so probable causes. These measures covered every single probable cause. Therefore, when the Japanese Ministry of Land, Infrastructure, Transport and Tourism, Transport Safety Bureau eventually pinpoints what caused the battery failures of JAL and another Japanese operator, all corrective actions will have already been completed. Simultaneously, these measures will prevent potential problems, such as smoke erupting from the battery or heat damage.
All problems causes of battery failure were identified.
Measures were implemented for all identified causes
Mechanism of Battery Failure
It was identified that the battery failures occurring aboard 787 of JAL and another Japanese operator were caused by the overheating of one of the 8 lithium -ion battery "cells" contained in the blue battery case, for some reason.
Boeing's analysis of mechanism of battery failure
1st and 2nd layer of improvements
All 18 measures implemented by Boeing are structured into multiple layers of redundancy, or backups, so that no single failure can cause an accident. In other words, even if one cell overheats despite preventive measures, there will be measures in place so that the heat will not be transmitted to other cells (second layer of improvements). The multiple layers of redundancy will significantly improve reliability of the battery system.
This second layer of improvements will significantly improve reliability of the battery system.
3rd layer of improvements in the unlikely event of a battery failure
Boeing identified all probably cases when establishing preventive measures. It also added a third layer of improvements to prevent trouble from spreading to the cabin in the unlikely event of a battery failure. These safety measures prevent fumes or heat from flowing into the cockpit or cabin, and eliminate any potential for fire in the electronic equipment bays where the batteries are installed.
Even if other cells overheat, the 3rd layer of improvements will prevent any fumes from flowing in the cabin.
The battery is now enclosed in sealed stainless steel. The enclosure isolates the battery from the rest of the equipment in the electrical equipment bays. Any fumes or heat that builds up in the enclosure due to overheating of cells will be sealed inside the enclosure, vented outside the airplane, and will not flow into the cockpit or cabin. The stainless steel enclosure is sealed. Therefore, in the event of a fire, the flames will die out naturally as there is not enough oxygen inside the enclosure.
Battery case enclosed in sealed stainless steel (sample for display)
The battery case (blue) is placed in a stainless steel enclosure (green), which can endure high pressure. The white cable is electric wiring. The tube to vent fumes or heat is installed in the rear.